It is understood that to unite two tubes of thermoplastic material, a sleeve of the same material is used in which the two end portions of the two tubes are inserted, that is to say the sleeve serves as a union, and in the internal superficial zone thereof there is imbedded a coil where the extremities are excited by an electrical current. When this source is electrified, the current flowing in the coil causes the fusion of the material in the internal superficial zones of the tube and the sleeve within which the tube is lodged. The two zones are subjected to a thermal dilation great enough so that the surfaces in fusion come into solid contact which assures their welding, when cooled, to form a solid block.
This simple process has a control difficulty however, since it is not visible from the exterior, but on the contrary, constitutes a progressive phenomenon, where the objective is the formation of a solid block between the tubes and their sleeve union, although this phenomenon can be surveyed by two physical indicies, the temperature and the volume.
Effectively, when the electrical current welding source is connected, the flow of current in the coil determines a sequence of events:
the turns of the coil being imbedded in the sleeve material, the heat developed by the flow of the current provokes the heating of the material in the surrounding zone, more and more approaching fusion and dilation up to its surface until it comes into contact with the tube exterior, onto which it exercises an increasing pressure;
next, the material of the tube enters into fusion thus insuring the welding under pressure of the tube with the sleeve.
if the current is not cut-off at this point, the dilation of the material in fusion will cause an increase in pressure in the welding zone, because the lateral zones on each side of the coil stay cold to avoid material loss and to assure the establishment of welding pressure.